Filament structure



y 29 1956 9. D. DOOUTTL 7 2,148,311

FILAMENT STRUCTURE Filed May 27, 1952 2 Sheets-Sheet l FA q INVENTOR HOWARD 0. DOOLITTLE y 29, 1956 H. D. DOOLITTLE 2,

F ILAMENT STRUCTURE Filed May 27, 1952 2 Sheets-Sheet 2 4? 4s 46 FIG. 4 42\ FIG. 5

INVENTOR HOWARD D. [2%LITTLE ATTORNEY United States Patent FILAMENT STRUCTURE Howard D. Doolittle, Stamford, Conn., assignor to Machlett Laboratories, Incorporated, Springdale, Conn., a corporation of Connecticut Application May 27, 1952, Serial No. 290,173

3 Claims. (Cl. 313264) This invention concerns a novel filament structure for use in high power electron tubes. More specifically, this invention concerns an elongated, multiple hairpin filament structure of great simplicity, which structure is unsupported by the electron tube envelope at one end.

Because the interelectrode spacing in power tubes is small relative to the length of their electrodes, the opportunity for the filament to short into the grid or the anode is great. Moreover, certain forces, such as electromagnetic and/or electrostatic forces, act upon the filament strands in such a manner that the strands tend to be driven apart and into the adjacent electrode. In order to provide a stable structure which will resist the effects of such forces, most kinds of prior art filament structures have been supported by the tube envelope at both ends. Support at one end has been furnished by support-conductors which are in turn supported by a stem press or other portion of the vacuum envelope. Support at the other end has frequently been furnished by a center mast which is supported by the stem press or otherwise. The center mast either provides means for keeping the filament tight through spring tensioning or else it provides guides or eyelets through which the filament strands pass, thereby permitting free expansion of the filament but only in a predetermined direction.

In addition to complicating the tube structure, a center mast is subject to failure due to sticking of the tensioning mechanism or adherence of the strands to the guides or eyelets. Furthermore, each of the relatively massive support members of the center mast is capable of cooling that portion of the filament which it contacts as well as adjacent filament portions. If the amount of this cooling were constant, the amount of reduced emission could be taken into consideration in designing a tube. However, the cooling, hence the emission, varies considerably due to expansion effects which, in some cases, cause the filament to be in various degrees of good and had contact with the support at diiferent times and entirely out of contact with the support at other times.

Attempts to eliminate the center mast have been successful in the so-called squirrel cage, free-hung filament, but elimination of the center mast in other types of filament structures has heretofore been considered impossible. For instance, elimination of the center mast in tubes employing high voltage, low current filaments has not been considered possible, despite the advantage to be had in this widely used type of filament.

My invention provides a filament structure, one end of which is unsupported by the vacuum envelope. The active portion of this filament structure may be described as of elongated, closely spaced saw-tooth or multiple hairpin shape. This active portion of the filament consists of refractory metal wire formed into a plurality of hairpin bights and arranged with all strand ends at that end of the filament which is supported by the vacuum envelope. In a preferred form of my filament structure, a single strand of refractory wire is formed into a plurality of reverse hairpin bights so that alternate bights 2,748,311 Patented May 29, 1956 define opposite ends of the active filament structure. The filament strand ends are in every case afiixed to support conductors which are, in turn, ultimately supported by the vacuum envelope. By my invention bights at the unsupported end of this filament structure are joined together through a non-conductive connection which 0pposes the electromagnetic and electrostatic forces tending to drive the otherwise unsupported bights apart and into the grid or anode structure. Thus, it is unnecessary with a structure of my invention -to supply .a center mast or any other support structure attached to the vacuum envelope for the purpose of lending support to the otherwise unsupported end of the filament structure.

For a better understanding of my invention reference is made to the following drawings:

flFig. 1 illustrates in partial section an electron tube which employs a three bight filament structure of my invention.

Fig. 2 is an elevational view of the active portion of the filament structure shown in Fig. 1, but rotated around the major tube axis.

Fig. 3 is an elevational view similar to that of Fig. l which shows an enlarged view of the filament in the same position seen in Fig. 1.

Fig. 4 shows in elevation a filament structure of my invention which employs more than three bights.

Fig. 5 shows the structure of Fig. 4 rotated 90 around the major tube axis.

Fig. 6 is .a plan view from above the unsupported end of filament structure of Figs. 4 and 5 showing one type of bracing which may be employed.

Fig. 7 is a view similar to Fig. 6 showing another type of bracing which may be substituted for that of Fig. 6.

Referring to Fig. l a high power triode electron tube is illustrated. The vacuum envelope of the tube is composed of a cylindrical glass section 10 closed at one end by a reentrant glass portion 11 and closed at the other end by a deep cup of copper 12 which provides the anode for the tube. Copper cup 12 may have a feather edge 12a at its lip in order to facilitate making the glass-tometal seal joining the two envelope portions 10 and 12. Anode heat dissipation may be easily accomplished in this tube by water-cooling or by air-cooling the outside surface of anode 12. Water cooling is facilitated by support of the tube in a water jacket at collar 12b.

A tubular glass extension 14 of the reentran't portion 11 of the vacuum envelope which extends inwardly into the vacuum envelope has sealed to its end a metallic grid support member 15 of generally conical shape. The grid support is joined to a ring 16 which in turn supports a plurality of posts 17 which parallel the tube axis. The active grid 18 consists of a helical winding of refractory metal wire around and atfixed to these support posts 17. The grid terminal 19 may be a small metallic cup whose lip is sealed vacuum tight to a portion of the vacuum envelope. Conductive connection member 20 will provide an electrical connection between support cone 15 and terminal member 19.

The active filament which is shown in elevation in Figs. 1, 2 and 3, is supported on a pair of rod-like support-conductor members 22 and 23. These members may be sealed through the vacuum envelop by using a conventional stem press if desired. However, as shown, support-conductor 23 is sealed vacuum tight through metallic cup member 24 which in turn is sealed vacuum tight to the envelope. Support-conductor 22, which is here shown broken before passing through its vacuum seal, may be sealed through the envelope in essentially the same manner as is support-conductor 23. The active filament consists of a single strand wire 25 of a refractory metal, such as tungsten, the strand ends of which are affixed, as by welding, to support conductors 22 and 233 Between the strand ends; the filament is formed into a multiple, hairpin. shapeproducing. an. active. filament. structure such that the length of the active structure is considerably larger than its overall diameter. In making. this; reverse hairpin. shaped .filament,.aaplurality; of bightson hairpin bendsare formed; The;bight ae1at= the; supported-end ofthe filament structure. is also:-sup.- portedultimately uponthe vacuum.envelope. As may: beseen in the structure. of .Figs. 2 and 3';thisr.is ad vantageously accomplished by means of wire connection mem ber 26, one end ofwhichisfastened around bight 25a and.the other. endof which isconnected .to insulator 27 as'by crimping in. place, using ametallicfstrap 28 wrapped aroundainsulator 27. As illustratedthe insulaton is in turn advantageously. connected. to" support-conductor members: 22;.and. 23 by relatively rigid wire-like connection means 29 andi30;-. In this instance-the connection means :29 andi3t) are fixed in .smalliholes parallel to the axis of cylindrical insulator 27. In other instances the insulator might .be supported upon someother means ultimately supported by the vacuum envelope or upon the-vacuum envelope itself. The illustrated means of accomplishing. the desired end is of particular advantage, however, because: of its simplicity. The bights25b at the opposite end of the filament structure are advantageously joined together through an insulating path by means of finewire ties 31 and 32, which are wrapped around the filament wire 25 and joined to cylindrical insulator 33'. It may benoted that insulator 33, like insulator 27, is advantageously made with a pair of-parallel holes extending through the insulator parallel to its axis. The wire ties 31 and 32 may each pass through and be fixed in one of these holes after making the tie connections to the filament strand. Because of the fineness of the tie wires 31 and 32, they will have but a small, almost negligible, cooling effect upon the filament. In general, the relatively-small means specifically described for supporting the filamentwill permit more nearly uniform filament temperature over most ofthe length of the filament. It is important, of course,- that wire members connected to the" filament wire 25 such as wires 26, 31, and-32, be of high-heat resistance, such as molybdenum, tungsten or tantalum. It isalso important that the insulator 32 have" high heat resistance and be incapable of retaining occluded gases which will be released when the-insulator is heated to the very high temperatures to which it mightbe subjected in its particular location adjacent the filament. I have found that insulators'composedofmaterials-such as alumina or'materials such-as quartz-are-satisfactory.

Ina-modifiedform ofthe filament, bightZSa'may -be connected to asupport-conductor which is se'aled through the vacuum envelope. A terminal'for such a lead might be used to divide the filament in half and'to place'the two-halves of the filament in parallel, or it'rnight 'bejcon nected to acentertap on the filament transformer. In' the-event that such an extralead isused, the filament may bemade in two separate hairpin shaped pieceseach having an end connected to the extra support-conductor or it may be a single strand, essentially like strand 25; In either event, the elements of my invention are still ap-' plicable.

The tube structure of Fig. l advantageously employs fiexible' connection means' to" extendsupport' conductors 22 and" 23' outside of the vacuum envelope. Filament terminals are advantageously mountedon a metallic base 36' of shallow cup-like'shape. One terminal 37 is insulated from the'base 36'by spoolshaped'dielectric means 38; The other terminal may be either similarly'insulated or con n ected to the base without insulation; Thelatter: choice is frequently taken in those cases where the ter minal 'in' question is grounded. The terminals are prong shape for easy couplingto circuitry.

The illustration of my invention as s' 'fe'cifically' described thus far, concerns' its'use' in its simplestform, whatmay be described as' a W-shapedfilament? When' higher emission is required of the filament, a longer strand bent. to form-more bights may be. employed...

Figs. 4 and 5 illustrate such a filament structure employing a single strand 40 of refractory metal filament wire having seven bights. The ends of this strand occur at the same end of the filament structure and are advantageously affixed to support-conductors 41 and 42 in the same manner employed in the structure of Figs. 2 and 3. In this instance, however,-thereare a plurality of bights 40a at the envelope supported end of thefilament. These bights may be individually connected as shown, by separate metallic conductors 43 to'asingle cylindricalinsulator 44 by means of similar, but separate, strap connectors 45% Means connecting the: cylindrical insulator 44 ultimately to thetube envelope may include wire-like members 46 and 47 joined to support-conductors 41 and 42. In the alternative a plurality dielectric members may be employed for each of the bights in the manner of the Fig. 1 co'nstruction which', insulators are ultimately supported 'onthe vacuum envelope; Various'oth'ermeans' of supporting the bights 40a ultimately onthe vacuum envelope will occur to one skilled in the art, andsuch means as are within the limits of the claimsare within' the scope ofmy invention;

Thebights 40b at the end of the filament unsupported bythee'nvelope may be connected together as shown in Fig. 6-or as shown in Fig. 7. If connections are made as shown in Fig. 6, the arrangement of eachconnectionis similar to that of Figs; 2 and 3, i; e., each adjacent pair of bights=is connectedby a diiferent insulator. Using this Fig. 6*construction, fine'wire' tie members 49- and 50=are-wrapped around the filament strand,- and these tie wires are in turn connected to and insulated from one" another by insulator 51, which may be acylinder of 'alu mina' as' previously. described. In the alternative, bracingof the type shown in- Fig. 7 using tie wires 53 and a single insulator 54 maybe employed: Both of'these' means for bracing the'otherwise unsupported ends of the .filament have merit. The construction of Fig. 6'

isadvantageous from the standpoint thatthe ties-join the-variousbights in the directions-of-the primary' forces acting to drive the filament bights apart.- On the otherhand, the bracing of Fig. 7 is simpler and more easily constructed than that of Fig. 6. It employs but onetie wire 53 for each ofthe bights 405, all of whichties conn'ectall the bights toa single -insula'tor-54; The'Fig;

7 construction will tend to'allow the'freeends of the bights to relocate themselves symmetrically around the insulator. Thisrelocation is smallin amountand'may not be of great disadvantage despitethe'fact that the' filament described is'basically anasymmetrical'typeof filament. Thefilamentrelocationpermitted by the Fig.

7 'typeabracing may make a slight ditference in *the *oper ating characteristics of the tube, however;

Theusefulnessof my novel filament structure is not confined to tubes like the one specifically clescrib'ed'and' additional supportaconductor members is possible, and

such members may be either'connected-to a bigh't at the envelope-supported end of the filament'or connected to filament strandends; In the latter case, more-than onefilament strandwill' be used. It is even conceivable that these-novel filament structures be used'in parallel within a vacuum envelope: All such. minor modifications iu structure are intended to be within the scope and spirit of my invention.

1 claim:

1. An electron tube embodying a pair of relatively closely spaced elongated electrodes, one of said electrodes comprising an elongated filament structure embodying a length of filament wire formed into a plurality of bi-ghts and having its ends both located at the same end of the filament structure and in spaced relation to the bights, and spacing means for retaining the bights in spaced relation from the other electrode and from each other cornprising an electrical insulating member disposed between the bights and connected to the respective 'bights by separate relatively fine high thermal resistance Wires secured directly to the filament wire, the connecting wires being of a diameter substantially smaller than the diameter of the filament wire.

2. An electron tube embodying a pair of relatively closely spaced elongated electrodes, one of said electrodes comprising an elongated filament structure embodying a length of filament wire formed into a plurality of bights and having its ends both located at the same end of the filament structure and in spaced relation to the high-ts, and spacing means for retaining the big-hts in spaced relation from the other electrode and from each other comprising an electrical insulating member disposed between the bights, and lengths of relatively high thermal resistance wire wound upon respective bights and secured to respective spaced portions of the insulating member whereby the temperature of the filament wire will be substantially unaffected by the spacing means.

3. An electron tube embodying a pair of relatively closely spaced elongated electrodes, one of said electrodes comprising an elongated filament structure embodying a length of filament wire formed into a plurality of big-hts and having its ends both located at the same end of the filament structure and in spaced relation to the bights, and spacing means for retaining the bights in spaced relation from the other electrode and from each other comprising a non-gas-emitting electrical insulating member disposed between the bights, and lengths of relatively high thermal resistance wire wound upon respective bights and secured to respective spaced portions of the insulating member whereby the temperature of the filament Wire will be substantially unaflfected by the spacing means.

Edison Sept. 11, 1888 Forst Nov. 5, 1901 

